DirectoryIndex set priority .html .php

I want index.html to be loaded first. I have a index.html (a holding page) and index.php (the Joomla CMS) and while I'm working on the site I want web users to see the index.html and only the client to see index.php, by explicitly typing it in as part of the URL.

I can do it on my local internal dev server, but not on my public ISPconfig server.